1. Field of the Invention
The present invention relates to the structure and fabricating method of a photoelectric device of Group III nitride semiconductor, and relates more particularly to the light emitting structure of a photoelectric device and the fabricating method thereof.
2. Description of the Related Art
To date, light emitting diodes made of gallium nitride material or Group III nitride semiconductor material are built upon a sapphire substrate mainly because the degree of lattice mismatch between sapphire and Group III nitride semiconductor material is low (normally, a buffer layer is still required to improve the mismatch therebetween). However, sapphire substrates have many disadvantages, such as high insulation characteristics, and due to such characteristics it is difficult for a light emitting diode made of Group III nitride semiconductor material to have a vertical conductive structure. Therefore, the technology using other substrate materials, for example silicon carbide material, to reduce such disadvantages continues to be developed. Due to its greater conductivity, silicon carbide can be used to produce a conductive substrate, and because the degree of lattice match between silicon carbide and Group III nitride active layer is low, using a buffer layer made of gallium nitride or aluminum gallium nitride, a Group III nitride semiconductor layer can be deposited on a silicon carbide substrate. Moreover, due to the high stability of silicon carbide, silicon carbide is becoming more important in such manufacturing processes. Although a Group III nitride semiconductor layer can be deposited on a silicon carbide substrate with the help of a buffer layer made of gallium nitride or aluminum gallium nitride, the degree of lattice match between a Group III nitride semiconductor material and silicon carbide, which is lower than the degree of lattice match between aluminum gallium nitride and silicon carbide, often causes defects in an expitaxial layer even where the buffer layer made of gallium nitride or aluminum gallium nitride is formed on a silicon carbide substrate, and furthermore, a silicon carbide substrate is more expensive.
FIG. 1A and FIG. 1B show a method of separating a thin film from a growth substrate, disclosed in U.S. Pat. No. 6,071,795. The method initially forms a separation region 12 and a silicon nitride layer 13 on a sapphire substrate 11, and then a bonding layer 14 is disposed on the surface of the silicon nitride layer 13. Next, with the help of the bonding layer 14, a silicon substrate 15 is bonded to the above-mentioned sapphire substrate 11 with a stacked-layer structure. A laser beam 16 penetrating the sapphire substrate 11 is applied on the separation region 12, and causes the separation region 12 to decompose. Finally, the remnant material of the decomposed separation region 12 is cleared to obtain a composite including the silicon substrate 15 and the silicon nitride layer 13. However, because the bonding layer 14 between the silicon substrate 15 and the silicon nitride layer 13 is dielectric, the composite cannot be a basis for building a vertical structure light emitting diode. Moreover, if the material for the bonding layer is disposed incorrectly or selected improperly, the bonding is affected, and defects are formed in the silicon nitride layer 13.
FIG. 2 shows a method of separating two layers of material from one another, disclosed in U.S. Pat. No. 6,740,604. The technology used for the disclosure related to FIG. 2 is similar to the technology for the disclosure related to FIGS. 1A-1B. A laser beam 23 is applied on the interface between a first semiconductor layer 21 and a second semiconductor layer 22, and initiates the decomposition of the second semiconductor layer 22 at the interface. Finally, the first semiconductor layer 21 is separated from the second semiconductor layer 22. The second semiconductor layer 22 can be the film layer formed on a substrate. In such process, a substrate replaces the first semiconductor layer 21, and then both are separated.
FIG. 3 shows a structure prior to separation of the substrate, disclosed in U.S. Pat. No. 6,746,889. The method initially grows several epitaxial layers, which comprise the first region 32 of a first conductivity type, a light-emitting p-n junction 33, and the second region 34 of a second conductivity type, on a substrate 31. Next, several sawing streets 36 are cut through the epitaxial layers of the first region 32, second junction 33 and region 34 to have a plurality of individual optoelectronic devices or dies 35 formed on the substrate 31. Thereafter, the second region 34 is bonded to a submount 37. As shown in the above mentioned prior art technology, a laser beam, in the same manner, penetrating the substrate 31 causes the substrate 31 to separate from the first region 32. Separated optoelectronic devices or dies 35 can be removed from the submount 37 and proceed through the packaging processes. Obviously, when the epitaxial layers are cut through, individual optoelectronic devices or dies 35 bonded to the submount 37 squeeze one another by external forces such that die cracks may occur.
FIG. 4 is a side view of the laser lift-off process for removing a sapphire substrate, disclosed in U.S. Pat. No. 6,617,261. A gallium nitride 42 is initially formed on a sapphire substrate 41, and then a plurality of trenches 44 are formed by etching process. Next, a silicon substrate 43 is bonded to the surface where the gallium nitride layer 42 is formed and then is etched to form the trenches 44. Thereafter, an ultraviolet excimer laser 45 emits a laser beam 46 onto the sapphire substrate 41. The laser beam 46 penetrates the transparent sapphire substrate 41 to cause the gallium nitride at the interface to decompose so as to obtain a silicon substrate 43 bonded with the gallium nitride layer 42. Any residual gallium metal on the surface of the gallium nitride layer 42 is removed by hydrochloric acid. The surface of the gallium nitride layer 42 needs to further repair for sequent epitaxial processes.
Conventional technologies use high-energy laser beams to separate substrates or light emitting dies. However, those technologies have low throughput and require expensive equipment to apply. Therefore, a new separation technology that has none of the above-mentioned issues, can guarantee the quality of produced light emitting dies, and can be applied for mass production is required by the market.